E-mail sent from someone pretending to be someone else is known as spoofing. Spoofing may take place in a number of ways. Common to all of them is that the actual sender's name and the origin of the message are concealed or masked from the recipient. For more information, visit http://en.wikipedia.org/wiki/E-mail_spoofing
One very common example of spoofing is when people receive e-mails in their inbox saying that a message could not be delivered to a recipient, even though they do not remember sending that message. In this case, someone has learned what your email is and sent spam e-mails pretending to be from your address and the e-mail address was incorrect and sent a bounce message back to your e-mail address.
The thumbnails below show some examples of spoofed e-mail messages: